The ones that contain the metro,prazi, and levamisole are pretty safe even with sensitive fish like discus, loaches and cories. Read the instructions and err on the side of to little rather than to much, you might try a 3/4 dose the first time. I have used it on clowns but never kept queens. If the fish that died looked skinny but fat at the same time, full bellied but a little thin elsewhere it was the worms. The brands seem pretty much the same.